Investments - Schedule of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, unrealized gains	$ 11	$ 0
Available-for-sale debt securities, unrealized losses	(89)	(409)
Cash equivalents, short-term and long-term investments, carrying value	195,942	255,760
Cash equivalents, short-term and long-term investments, fair vale disclosure	195,864	255,351
Money market funds, included in cash equivalents
Debt Securities, Available-for-sale [Line Items]
Cash equivalents at carrying value	41,434	21,059
Money market funds, included in cash equivalents	41,434	21,059
Short-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, unrealized gains	3	0
Available-for-sale debt securities, unrealized losses	(81)	(296)
Total cash equivalents and short-term investments, carrying value	189,952	233,448
Total cash equivalents and short-term investments, fair value	189,874	233,152
Short-term Investments \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	65,887	58,136
Available-for-sale debt securities, unrealized gains	2	0
Available-for-sale debt securities, unrealized losses	(39)	(64)
Available-for-sale debt securities, fair value	65,850	58,072
Short-term Investments \| U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	53,765	111,049
Available-for-sale debt securities, unrealized gains	1	0
Available-for-sale debt securities, unrealized losses	(8)	(101)
Available-for-sale debt securities, fair value	53,758	110,948
Short-term Investments \| Government agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	28,866	43,204
Available-for-sale debt securities, unrealized gains	0	0
Available-for-sale debt securities, unrealized losses	(34)	(131)
Available-for-sale debt securities, fair value	28,832	43,073
Long-term Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	5,990	22,312
Available-for-sale debt securities, unrealized gains	8	0
Available-for-sale debt securities, unrealized losses	(8)	(113)
Available-for-sale debt securities, fair value	5,990	22,199
Long-term Investments \| Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	2,001	7,117
Available-for-sale debt securities, unrealized gains	0	0
Available-for-sale debt securities, unrealized losses	(8)	(16)
Available-for-sale debt securities, fair value	1,993	7,101
Long-term Investments \| U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Available-for-sale debt securities, amortized cost basis	3,989	15,195
Available-for-sale debt securities, unrealized gains	8	0
Available-for-sale debt securities, unrealized losses	0	(97)
Available-for-sale debt securities, fair value	$ 3,997	$ 15,098

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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